In the last thirty years satellite services have come to play an increasingly important role in telecommunications. Antenna design is an important part of a satellite system. One type of satellite antenna suitable for satellite communications is the quadrifilar helix first disclosed by Kilgus (Resonant Quadrifilar Helix Design, Microwave Journal, December 1970). This antenna consists of four helical windings fed in phase quadrature.
This configuration exhibits many performance characteristics well suited to satellite communications, namely, a hemispherical omnidirectional radiation pattern with excellent circular polarization throughout the radiation pattern, as well as compactness and simplicity. Omnidirectional coverage is desirable to allow the earth terminal to see the satellite regardless of its relative orientation to the satellite. The geometries of this design employ a resonant matching network. Hence the operating bandwidth is typically narrow.
Recently, higher performance mobile satellite services have emerged which require increased antenna gain. Due to the location of population of the earth, the areas that many service providers have allocated high priority to are in the northern hemisphere. As a result the important converge profile is located at 20 to 60 degrees in elevation with respect to the earth terminal.
In prior art, the increase in gain at these elevations is accomplished by increasing the height of the quadrifilar helix antenna. The compact quadrifilar helix antenna, which can exhibit nearly uniform hemispherical gain, does not provide enough gain at 20 to 45 degrees in elevation for the high performance systems. More energy can be directed to the low elevation angles by increasing the height of the antenna. The tradeoff between size and performance leads to a significant increase in the height of the antenna. As low profile structures are highly desirable for mobile communications, this is a considerable disadvantage.
An object of the invention is to overcome this disadvantage.